Compressed gas methods and devices for generating acoustic pulses in a medium such as earth formations, soil, marsh land or swamps are described in U.S. Pat. Nos. 4,051,918 Rogers and 3,997,021 and 3,779,335 Chelminiski. Similar methods and devices are used for generating pulses in a water medium, as described in U.S. Pat. Nos. 4,049,078 Paitson, 4,047,591 Ward, 4,034,827 Leerskov, 3,750,097 Havlik, 3,653,460 Chelminski, 3,506,085 Loper, 3,434,562 Johnson, 3,397,755 Loper, and 3,379,273 Chelminski.
A typical gun comprises a valve chamber adapted to confine a charge of compressed gas (ordinarily air) at a high pressure. There is at least one passage for the compressed gas from the valve chamber into the surrounding medium (e.g., water). This passage is closed while the pressure (as from a compressor on a surface vessel) is built up in the chamber. The passage is opened when the gun is "fired", allowing the compressed gas to expand out of the chamber and into the surrounding medium.
Since the discharge of a seismic air gun occurs with the release of substantial explosive force, and since the gun is usually adapted for use under water, down a borehole or the like, the firing is ordinarily effected in response to a pneumatic or electrical signal originating at a remote location. After the gun is fired, the passage from the valve chamber into the medium must be closed and the chamber must be recharged to the high pressure before the gun can be fired again. In some cases, the individual steps in the sequence of charging the chamber, opening the passage to discharge the gun, and reclosing the passage to allow recharging, have been effected or initiated in response to separate remotely-originating signals, as disclosed in U.S. Pat. Nos. 3,506,085 and 3,397,755 Loper and 3,379,273 Chelminski. In most cases, however, the design of the gun is such that the sequence is semi-automatic, with the gun being discharged in response to a discrete firing signal or in response to a drop in the pressure of the air line to the compressor.
To open and close the passage between the valve chamber and the surrounding medium, many seismic air guns utilize a shuttle member having an actuating piston or the like on one end, and a valve portion on the other end for opening and closing one or more port apertures. Guns using the shuttle arrangement are disclosed in U.S. Pat. Nos. 4,047,591; 4,034,827; 3,997,021 and 3,653,460 supra; 3,638,752 Wakefield; 3,506,085; 3,434,562; 3,397,755 and 3,379,273 supra; 3,379,272 Brooks and 3,276,534 Ewing. U.S. Pat. No. 3,750,097 supra discloses a somewhat different, floating-piston valve arrangement.
Other seismic air guns utilize axially-movable sleeve valves that surround a portion of the valve chamber, as disclosed in U.S. Pat. Nos. 4,051,918; 4,049,078 and 3,638,752 supra. U.S. Pat. No. 4,049,078 discloses a motor and cam arrangement for actuating the sleeve valve. The other two patents disclose sleeve valves activated by the pressure of the compressed gas on axial thrust-producing areas of the sleeve valve members. A typical embodiment of the present invention utilizes a sleeve valve arrangement that is generally of the latter type.
The operators of the prior art seismic air guns have been able to exercise some measure of control over the amplitude and waveshape of the generated pulses insofar as they could select the pressure within the valve chamber and the depth (in water) at which the gun was fired. Some guns have also been designed with interchangeable parts, whereby a selection of a parameter such as the effective volume of the air-confining chamber could be made (with some inconvenience) as a means of influencing the characteristics of the generated acoustic pulses. Otherwise the pulse characteristics were effectively predetermined by the design of the gun.